High-Efficiency, Wide Dynamic Range Dimming for Solid-State Lighting

ABSTRACT

A solid-state lighting system includes a light-emitting device connected in series with a variable dim setting resistor, an adjustable regulator, and a current sensor disposed in a feedback loop between an adjust input of the adjustable regulator and the variable dim setting resistor. The voltage dropped across the variable dim setting resistor is forced to remain constant irrespective of the resistance setting of the dim setting resistor. The current sensor measures or senses the current flowing through the variable dim setting resistor and causes the adjustable regulator to adjust its output voltage and the forward voltage drop across the light-emitting device so that the current flowing through the light-emitting device matches the current set by the variable dim setting resistor. Controlling the current in this manner allows dimming to be performed over an extremely wide dimming range, to very low light levels, and without producing light flicker at any dim level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/620,712, filed on Feb. 12, 2015, which claims the benefit of U.S.Provisional Patent Application No. 61/944,149, filed on Feb. 25, 2014.

FIELD OF THE INVENTION

The present invention relates in general to solid-state lighting (SSL)systems and in particular to methods and apparatus for performingdimming in SSL systems over an extremely wide dimming range and to verylow dimming levels.

BACKGROUND OF THE INVENTION

Solid-state lighting (SSL) is rapidly becoming commonplace in many areasof modern society. SSL employs light-emitting diodes (LEDs), which areinexpensive, durable, small in size, energy-efficient, and long-lasting.This desirable combination of attributes is not present in moreconventional lighting sources, such as incandescent and fluorescentlighting sources.

In some circumstances it is necessary or desirable to control dimming ofthe light produced by a lighting system. In an SSL system, dimming istypically performed using a technique known as pulse width modulation(PWM). FIG. 1 is a drawing showing the salient components of a PWM-baseddimming system 100 typically used to perform dimming in an SSL system.The PWM-based dimming system 100 comprises an LED driver 102, an LED 104(or string of LEDs connected in series), a transistor 106, and acurrent-limiting resistor 108. The LED driver 102 generates a PWMcontrol signal which is used to drive the gate of the transistor 106.The PWM control signal comprises a sequence of pulses which determinewhether the transistor 106 is turned ON or OFF. Since the transistor 106is in series with the LED 104, turning the transistor 106 ON and OFFintermittently interrupts the current flowing through the LED 104 and,consequently, intermittently interrupts the light that the LED 104emits. The durations (i.e., “widths”) of the pulses in the PWM controlsignal are controlled (i.e., “modulated”) by the LED driver 102,according to the setting of a dim control signal DIM applied to a dimcontrol input of the LED driver 102. As the dim control signal DIM islowered to effect dimming, the LED driver 102 responds by reducing thewidths of the pulses in the PWM control signal. The reductions in thewidths of the pulses results in a lower percentage of time during whichthe LED 104 conducts and emits light. On average, this result isperceived by the human eye as dimming.

In some lighting applications the light produced by a lighting systemmust be dimmable over a wide dynamic range and to very low light levels.In a movie theater, concert hall, or opera house, for example, it isoften desirable to have a dimming dynamic range of 1000:1 or greater.When a PWM-based dimming system is employed to control the dimming, theduty cycle of the PWM control signal must therefore drop to as low as0.1%, in order to realize the 1000:1 dimming dynamic range.Additionally, the frequency of the PWM control signal must be designedand set high enough to avoid undesirable light flicker. Unfortunately,due to the significant capacitance that is normally present at the inputof most commercially available transistors 106, the PWM-based dimmingapproach is unable to respond properly to a PWM control signal having aduty cycle as low as 0.1% and a frequency that is high enough to avoidflicker, particularly at low light levels. This problem makes PWM-baseddimming an unsatisfactory solution to achieving dimming over a widedynamic range and to very low light levels.

It would be desirable, therefore, to have an SSL dimming system that iscapable of realizing dimming over a wide dynamic range and to very lowlight levels, that is energy efficient, and that does not produce lightflicker, especially at low light levels.

BRIEF SUMMARY OF THE INVENTION

Systems and methods for controlling dimming in solid-state lighting(SSL) systems are disclosed. An exemplary SSL lighting system forcontrolling dimming includes a variable dim setting resistor, one ormore light-emitting devices, an adjustable regulator, and a sensor formeasuring or sensing a current flowing through the variable dim settingresistor. The variable dim setting resistor is coupled in series withthe one or more light-emitting devices and is configured to receive adim control signal which sets and controls the resistance of thevariable dim setting resistor. The adjustable regulator is configured togenerate an adjustable and regulated power supply for powering the dimsetting resistor and the one or more light-emitting devices. The sensoris disposed in a feedback loop between the variable dim setting resistorand an adjust input of the adjustable regulator and in one embodiment ofthe invention comprises an operational amplifier (op-amp) circuit whichoperates to maintain a constant voltage across the dim setting resistor,independent of the value of the dim control signal and resistance of thevariable dim setting resistor. Upon the sensor measuring or sensing thatthe current flowing through the dim setting resistor has decreased (thedecrease indicating that the variable dim setting resistor has beenadjusted higher), the op-amp circuit generates an adjust signal for theadjustable regulator, causing the adjustable regulator to reduce thevoltage it generates and supplies. The lower supply voltage causes thevoltage drop across the one or more light-emitting devices to alsodecrease, resulting in a dimming of the light emitted by the one or morelight-emitting devices. Test measurements have shown that the SSLsystems of the present invention are capable of dimming over anextremely wide dimming range. For example, measurements have shown thatnot only are the SSL systems capable of a dimming ratio of greater than1000:1, they have also shown that a dimming ratio exceeding 20,000:1 canbe achieved.

Further features and advantages of the invention, including descriptionsof the structure and operation of the above-summarized and otherexemplary embodiments of the invention, will now be described in detailwith respect to accompanying drawings, in which like reference numbersare used to indicate identical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a prior art solid-state lighting (SSL)system that performs dimming using pulse-width modulation;

FIG. 2 is a schematic drawing of an SSL system configured to performdimming, in accordance with an embodiment of the invention;

FIG. 3 is a schematic drawing illustrating how the sensor of the SSLsystem in FIG. 2 may be implemented using an operational amplifier, inaccordance with an embodiment of the invention;

FIG. 4 is a schematic illustrating how the adjustable regulator of theSSL system in FIG. 2 may be implemented using a switching regulator, inaccordance with an embodiment of the invention;

FIG. 5 is a schematic illustrating how the adjustable regulator of theSSL system in FIG. 2 may be implemented using a linear regulator, inaccordance with an embodiment of the invention;

FIG. 6 is a schematic drawing illustrating how the variable dim settingresistor of the SSL system in FIG. 2 may be implemented using a resistorbank and resistor select circuit, in accordance with an embodiment ofthe invention; and

FIG. 7 is a block diagram of an integrated circuit implementation of anSSL system with dimming capability, in accordance with an embodiment ofthe invention.

DETAILED DESCRIPTION

Referring to FIG. 2, there is shown a solid-state lighting (SSL) system200 having extremely wide-range dimming capability, according to anembodiment of the present invention. The SSL system 200 comprises anadjustable regulator 202, a current sensor 204, a light-emitting device206, and a variable dim setting resistor R_(SET) 208. In the descriptionthat follows the SSL system 200 is described as controlling dimming of asingle light-emitting diode (LED) 206. However, those of ordinary skillin the art will appreciate and understand that the SSL system 200 can beused to control dimming of a plurality of LEDs (e.g., a string oflight-emitting devices). Further, the invention is not limited tocontrolling dimming of LEDs. It can be used or adapted to controldimming of other types of light-emitting devices, such as laser diodes,for example.

As shown in FIG. 2, the adjustable regulator 202 has an input powerterminal (IN) that is configured to receive an input voltage V_(in) froman external power supply (not shown), an output terminal (OUT) thatprovides a regulated and adjustable output voltage V_(OUT), and anadjust terminal (ADJ) which is configured to receive an adjust signalfrom the output of the current sensor 204. The regulated output voltageV_(OUT) serves as a supply voltage for the LED 206, which, as shown, isconnected between the output terminal of the adjustable regulator 202and a first end of the dim setting resistor R_(SET) 208. A second end ofthe dim setting resistor R_(SET) 208 is connected to ground. The currentsensor 204 is configured in a feedback loop between the first end of thedim setting resistor R_(SET) 208 and the adjust terminal ADJ of theadjustable regulator 202.

The current sensor 204 may be implemented in various ways. In oneembodiment of the invention depicted in FIG. 3, for example, it isimplemented using an operational amplifier (op-amp) circuit 310. Duringoperation, the adjustable regulator 202 serves to increase or decreaseits output voltage V_(OUT) and the forward voltage drop across the LED206, depending on the resistance setting of the dim setting resistorR_(SET) 208 (as set by a dim control signal DIM). A change in theresistance setting of the dim setting resistor R_(SET) 208 results in achange in the current I_(OUT) flowing through it. The range of currentflowing through the LED 206 therefore depends on the adjustable range ofthe dim setting resistor R_(SET) 208. In one embodiment of theinvention, for example, the dim setting resistor R_(SET) 208 isadjustable over a range of about 0.5Ω to 10 kΩ. Due to the presence ofthe op-amp circuit 310, the voltage dropped across the dim settingresistor R_(SET) 208 is forced to be maintained at substantially thesame voltage as the reference voltage V_(REF), regardless of theresistance setting of the dim setting resistor R_(SET) 208. As theresistance of the dim setting resistor R_(SET) 208 is adjusted higher(to increase dimming), the current I_(OUT) flowing through the dimsetting resistor R_(SET) 208 decreases. The op-amp circuit 310 sensesthe decrease in current I_(OUT) and causes the adjustable regulator 202to lower its output voltage \V_(OUT) so that the forward voltage dropacross the LED 206 tracks and corresponds to a current in the LED's I-Vcharacteristics that matches the reduced current flowing through the dimresistor R_(SET) 208. (The current flowing through the dim settingresistor R_(SET) 208 must always match the current flowing through theLED 206 since the dim setting resistor R_(SET) 208 and LED 206 areconnected in series.) The reduced current flowing through the LED 206results in a corresponding reduction in light emitted by the LED 206,thereby producing the desired dimming effect. Conversely, when the valueof the dim setting resistor R_(SET) 208 is adjusted lower to increasebrightness (i.e., reduce dimming), the current I_(OUT) increases and theadjustable regulator 202 increases its output voltage V_(OUT) so thatthe forward voltage drop across the LED 206 and the current flowing itincrease to match the current set by the dim setting resistor R_(SET)208. The increased current I_(OUT) results in more light being emittedby the LED 206 and consequently an increase in brightness of the lightemitted by the LED 206.

The adjustable regulator 202 can be implemented in various ways. It canbe implemented using a switching converter 402, as illustrated in FIG.4. Alternatively, it can be implemented using a linear regulator 502, asillustrated in FIG. 5.

The SSL systems disclosed herein are capable of achieving dimming over awide dynamic range (up to 1000:1 or even greater). In variousembodiments of the invention a dimming ratio exceeding 20,000:1 has evenbeen realized. The resistance range required of the dim setting resistorR_(SET) 208 is further easily realizable using standard integratedcircuit (IC) manufacturing processes. As an example, using a referencevoltage V_(REF)=100 mV and for a 1000:1 dimming range and an LED thatconducts 350 mA at full brightness (typical of a high-brightness LED(HB-LED)) and 350 μA at maximum dimming, the minimum value R_(SET)(min)of the dim setting resistor R_(SET) 208 would be 0.28 ohms (fullbrightness) and the maximum value R_(SET)(max) of the dim settingresistor R_(SET) 208 would increase to 333 ohms (maximum dimming). Thisrange of resistance values is well within the range of resistor valuesthat can be manufactured using standard and readily available ICmanufacturing processes.

In one embodiment of the invention the dim setting resistor R_(SET) 208is fabricated and implemented using a plurality ofmetal-oxide-semiconductor field-effect transistors (MOSFETs) 602arranged in what may be referred to as a “resistor bank” 604, asillustrated in FIG. 6. The resistor bank 604 is controlled by a resistorselect circuit 606. Depending on the DIM control setting applied to theresistor select circuit 606 and the value of the dim setting resistorR_(SET) needed to achieve a certain dimming level, one or a plurality ofthe MOSFETs 602 in the resistor bank 604 is/are selected and biased intheir triode region of operation so that they can serve as reliable,constant-valued resistors. When a plurality of MOSFETs 602 is selected,the MOSFETs 602 are configured in parallel and, therefore, a lowerresistance for R_(SET) is established. Conversely, when a fewer numberof MOSFETs 602 is selected the resistance increases and a higherresistance for R_(SET) is established. Low power dissipation in the dimsetting resistor R_(SET) 208 allows reliable and straightforwardimplementation. From the earlier example provided in paragraph [00020]above, the maximum power dissipation is only (0.1²/0.28)=36 mW.

In the exemplary resistor bank 604 just described, it is assumed thateach MOSFET 602 has the same resistance value and that a plurality ofMOSFETs 602 is selected and configured in parallel to realize differentvalues of R_(SET). However, that need not be the case. For example, theresistor bank 604 can be alternatively designed to include a pluralityof MOSFETs, each having a different resistance value (with differentlength-to-width L/W ratios) and/or the resistor bank 604 and resistorselect circuit 602 can be configured such that only a single one of thedifferent-sized MOSFETs/resistances is selected for each dim setting. Itshould also be mentioned that whereas MOSFETs 602 are used to implementthe various resistors of the resistor bank 604, rather than usingMOSFETs 602, the resistors of the resistor bank 604 can be alternativelyimplemented using any other suitable resistor manufacturing techniqueemployed in IC fabrication processing.

The other components of the SSL system 200, including the adjustableregulator 202 and current sensor 204, may also be manufactured accordingto standard and readily available semiconductor manufacturing processes.In one embodiment of the invention illustrated in FIG. 7, all or mostall of the components of the SSL system 200 are fabricated in a singleIC 700. The IC 700 is assembled and packaged with an input power pin orterminal 702 for receiving the DC input voltage V_(in) from an externalpower supply 701, pins or terminals 704 and 706 for connecting the LED206 (or LED string), and an input pin or terminal 708 for receiving thedim control signal DIM. The IC 700 may also include a dim controlinterface 710 for interfacing with an external controller 712, which maybe an analog or digital, depending on the design. For example, the dimcontrol interface 710 may be a controller that responds to a device assimple as an analog switch or may be a more complex digital interfacethat is configured to respond to a digital signal, such as the digitaloutput of an analog-to-digital converter, microcontroller or digitalprocessor, for example.

In addition to being capable of dimming over a wide dynamic range and tovery low light levels, the SSL systems of the present invention produceno flicker at any dim level. As explained above in the Background of theInvention, prior art SSL systems typically employ pulse-width modulationto perform dimming, which results in light flicker that is particularlyperceptible at lower light levels. Flicker is not merely a slightnuisance. It is known to cause headaches, impaired visual performance,and diminished ocular motor control, and in some individuals it can eveninduce or increase the risk of epileptic seizures.

The SSL system of the present invention is also very energy efficient.For a maximum LED current of, for example, 350 mA (typical for anHB-LED), the dim-setting resistor maximally adds only 0.1×0.35=35 mW(assuming V_(REF)=100 mV) to the overall system power dissipation. Powerdissipation can be further reduced by using a lower reference voltageV_(REF). For example, for VREF=40 mV, the added power dissipation fallsto only 14 mW.

Finally, only a very small control voltage overhead is needed to achievedimming over a wide dynamic range. For instance, in the example providedabove the control voltage overhead is only the small reference voltageV_(REF) that appears across the dim-setting resistor R_(SET) 208.Assuming V_(REF)=100 mV (a value of V_(REF) in one embodiment of theinvention), the control voltage overhead is less than 3% of the typical3.5 V forward voltage drop of an HB-LED.

While various exemplary embodiments of the present invention have beenillustrated and described in detail above, those of ordinary skill inthe art will appreciate and understand that various changes in form anddetail may be made without departing from the true spirit and scope ofthe invention. The scope of the invention should therefore not berestricted to the specifics of the exemplary embodiments described butinstead determined by the words of the appended claims and the fullscope of equivalents to which such claims are entitled.

What is claimed is:
 1. A solid-state lighting (SSL) system, comprising:a string of one or more light-emitting devices; a dim-setting resistorconnected in series with the string of one or more light-emittingdevices, the dim-setting resistor having a variable resistance thatdetermines a magnitude of current flowing through the string of one ormore light-emitting devices and, consequently, the brightness of lightemitted by the string of one or more light-emitting devices; a voltageregulator configured to produce and apply a regulated voltage across theseries-connected dim-setting resistor and string of one or morelight-emitting devices; and a feedback system coupled between thedim-setting resistor and an adjust input of the voltage regulator, thefeedback system having a first input terminal configured to receive avoltage representative of the current flowing through the string of oneor more light-emitting devices, a second input terminal configured toreceive a fixed reference voltage, and an output terminal coupled to avoltage adjust input terminal of the voltage regulator.
 2. The SSLsystem of claim 1, wherein neither the regulated voltage produced by thevoltage regulator nor the current flowing through the string of one ormore light-emitting devices is interrupted to effect dimming of thelight emitted by the string of one or more light-emitting devices. 3.The SSL system of claim 1, wherein the feedback system comprises anoperational amplifier having inverting and non-inverting input terminalsand one of the inverting and non-inverting input terminals is configuredto receive the voltage representative of the current flowing through thestring of one or more light-emitting devices while the other of theinverting and non-inverting input terminals is configured to receive thefixed reference voltage.
 4. A method of controlling dimming in asolid-state lighting (SSL) system, comprising: directing a continuousand uninterrupted current through a string of one or more light-emittingdevices and a dim-setting resistor connected in series with the stringof one or more light-emitting devices; increasing a resistance of thevariable dim-setting resistor to reduce a magnitude of the continuousand uninterrupted current being directed through the string of one ormore light-emitting devices and dim light being emitted by the string ofone or more light-emitting devices; and regulating a voltage appliedacross the series-connected string of one or more light-emitting devicesand dim-setting resistor, wherein regulating the voltage applied acrossthe series-connected string of one or more light-emitting devices anddim-setting resistor includes continually forcing a voltage drop acrossthe dim-setting resistor to be maintained at a fixed reference voltage,irrespective of the resistance setting of the variable dim-settingresistor and irrespective of the magnitude of the continuous anduninterrupted current being directed through the string of one or morelight-emitting devices.
 5. The method of claim 4, wherein regulating thevoltage applied across the series-connected string of one or morelight-emitting devices and dim-setting resistor comprises: applying avoltage representative of the magnitude of the continuous anduninterrupted current to a first input terminal of an operationalamplifier; applying the fixed reference voltage to a second inputterminal of the operational amplifier; and applying an adjust voltageproduced at an output terminal of the operational amplifier to an adjustvoltage input of a voltage regulator that is configured to regulate thevoltage applied across the series-connected string of one or morelight-emitting devices and dim-setting resistor.
 6. A method of dimminglight emitted by a lighting system, comprising: applying a voltageacross a series-connected string of one or more light-emitting devicesand a dim-setting resistor; increasing a resistance of the dim-settingresistor to lower a current flowing through the string of one or morelight-emitting devices and thereby dim light emitted by the string ofone or more light-emitting devices; and as the resistance of thedim-setting resistor is increased to lower the current flowing throughthe string of one or more light-emitting devices, decreasing a voltageacross the string of one or more light-emitting devices while forcing avoltage across the dim-setting resistor to remain at a fixed voltage. 7.The method of claim 6, wherein neither the voltage applied across theseries-connected string of one or more light-emitting devices anddim-setting resistor nor the current flowing through the string of oneor more light-emitting devices is interrupted to effect dimming of thelight emitted by the string of one or more light-emitting devices.